1. Field
Methods and apparatuses consistent with exemplary embodiments relate to a method and apparatus for scheduling a downlink packet in a wireless communication system, and more particularly, to a method and apparatus for scheduling a downlink packet, wherein a wireless communication base station which has to send a plurality of pieces of user traffic to each user receiver (user equipment) through shared radio resources selects a user traffic flow to be transmitted in a downlink for each point of time.
In other words, exemplary embodiments relate to a method and apparatus for scheduling a downlink packet, wherein a wireless communication base station which has to send a plurality of pieces of user real-time traffic and non-real-time traffic in a downlink by using a limited number of physical resource blocks (PRBs), selects a user traffic flow that will use PRBs in the downlink for each point of time of scheduling.
2. Description of the Related Art
In general, a downlink scheduler (i.e., a packet scheduler) plays a very important role in determining system performance in a recent wireless communication system using a packet switching method, such as 3rd generation partnership project (3GPP) long-term evolution (LTE).
This is because the amount of downlink transmission resources is limited, whereas if a proper scheduling method is not used, there is a danger of deteriorated quality of service (QoS) for corresponding traffic because downlink traffic experiences packet delay and a buffer overflow due to its burst characteristic.
FIG. 1 shows a construction according to an embodiment for the downlink of a wireless communication system in which exemplary embodiments are applied.
In general, an element that plays an important role in order to provide QoS for user traffic, is the downlink scheduler (i.e., the packet scheduler) 13 of a wireless base station. User traffic flows (downlink traffic) that have to be transmitted by the wireless base station in a downlink, as shown in FIG. 1, is stored in buffer memory 12 in which each traffic flow is designated by a traffic classifier 11. The downlink scheduler (i.e., the packet scheduler) 13 determines which packets of traffic flow, from among all the traffic flows, will be transmitted to user equipment 14 through wireless channels for every transmit time interval (TTI).
For reference, NPRB PRBs are present within one transmit time interval (TTI).
Meanwhile, since the total data throughput of a system and QoS for each traffic flow are determined depending on what criteria are applied to the process of selecting a traffic flow, there is a need for a scheduling method capable of evenly satisfying a QoS level requested by each user traffic flow and also maximizing the total data throughput of a system.
Representative scheduling methods, from among a variety of scheduling methods proposed so far in order to satisfy the requirements, include a round-robin method, a maximum through (MT) method, and a proportional fair (PF) method.
Each of the scheduling methods is described in more detail below.
First, the round-robin method is a method of selecting a traffic flow to be transmitted through each PRB according to specific sequence (e.g., in sequence of increasing traffic flow index) if N traffic flows are present. In this round-robin method, radio resources can be fairly distributed into all traffic flows because a transmission opportunity corresponding to the share of each traffic flow is assigned to each of all the traffic flows, but the total data throughput of a system is not optimized because the capacity of a radio link that is different for each user is not taken into consideration.
Next, the MT method is a method of selecting a combination of a PRB and a traffic flow through which the greatest amount of data can be transmitted because the traffic flow, from among traffic flows including packets to be transmitted in a buffer, has the best instantaneous channel capacity of a corresponding wireless channel and assigning an opportunity capable of sending the packets through the PRB to the traffic flow, as in [Equation 1] below.
                              (                                    s              ^                        ,                          n              ^                                )                =                  arg          ⁢                                          ⁢                                    max                              s                ,                n                                      ⁢                          D                              s                ,                n                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
Here, DS,n is the number of bits that is assumed in a system and that can be transmitted by a traffic flows through a PRB n and is determined based on a channel state information (CSI) value of the corresponding PRB.
This MT method is advantageous in that it can maximize the total data throughput of a system because a combination of a PRB and a traffic flow through which the greatest amount of data can be transmitted, but there is a limitation in which the QoS or fairness of traffic flows is not taken into consideration.
Next, the PF method is a method of calculating a ratio of the number of bits DS,n that can be transmitted through a corresponding PRB for each traffic flow and the total data throughput RS of a traffic flow that has been transmitted according to results allocated right before a PRB as a priority function and selecting a traffic flow having the greatest priority function, from among the calculated priority functions, so that the traffic flow is transmitted through the corresponding PRB, as in [Equation 2] in assigning a PRB.
                              (                                    s              ^                        ,                          n              ^                                )                =                  arg          ⁢                                          ⁢                                    max                              s                ,                n                                      ⁢                                          D                                  s                  ,                  n                                                            R                s                                                                        [                  Equation          ⁢                                          ⁢          2                ]            
This PF method can be considered as a method of slightly compromising the total data throughput and improving fairness as compared with the MT method.
The limitation of the PF method, however, is that it is difficult to provide QoS for a delay-sensitive traffic flow because packet delay is not taken into consideration like in the round-robin method.